Laser satellite communication system

ABSTRACT

A laser communication system adapted for use in a satellite communication system. The satellite carries a laser communication system. The laser communication system includes a plurality of active and passive optical elements packaged in a monolithic, or single block, structure for interfacing between a focusing beam director of the satellite and laser transmitters/receivers of the laser communication system. Laser energy is directed between the beam director and the transmitters/receivers by the active and active optical elements, such laser energy passing through the monolithic structure solely as collimated light. In this way, diffraction relay elements, such as focusing lenses, and there concomitant alignment requirements, are eliminated from the monolithic structure.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to satellite communicationsystems and more particularly to laser satellite communication systemswherein data is transmitted to, and/or from a satellite using lasers.

[0002] As is known in the art information is sometimes transmittedbetween various locations on the earth by routes which includesatellites. More particularly, in the routing process, information maybe transmitted from a ground station along the route to a satellite. Thereceiving satellite may, in some arrangements, retransmit theinformation to a remote ground station along the route. In otherarrangements, the receiving satellite may be retransmit the informationdirectly to another satellite along the route, which, in turn may itselfretransmit to another satellite, or to a remote ground station. Thetransmission path, or data link, directly between a pair of satellitesis sometimes referred to as an inter-satellite cross link in the routingprocess. While transmission of data between the ground station andsatellite is typically by radio frequency (RF) energy, the use of laserenergy, at least for communication between satellites (i.e., for theinter satellite cross links) offers distinct advantages over radiofrequency (RF) systems, particularly for satellite cross links. Theseadvantages include the potential for a great reduction in weight, powerfor a given data rate, lack of optical spectral congestion and frequencyallocation requirements, immunity to electromagnetic interference,co-located transmitters and RF jammers.

SUMMARY OF THE INVENTION

[0003] In accordance with the present invention, a laser communicationsystem adapted for use in a satellite communication system is provided.The satellite carries a laser communication system. The lasercommunication system includes a plurality of active and passive opticalelements packaged in a monolithic, or single block, structure forinterfacing between a focusing telescope of the satellite and lasertransmitters/receivers of the laser communication system. Laser energyis directed between the telescope and the transmitters/receivers by theactive and active optical elements, such laser energy passing throughthe monolithic structure solely as collimated light. In this way,diffraction relay elements, such as focusing lenses, and theirconcomitant alignment requirements, are eliminated from the monolithicstructure.

[0004] In accordance with another feature of the invention, themonolithic structure is configured to provide all optic axes between thetelescope and a laser transmitter/receivers in substantially a commonplane. More particularly, the laser communication system includes aacquisition laser transmitter and an acquisition receiver used to enablethe satellite to link up with another satellite, or ground station,during an acquisition mode, and a communication laser transmitter and ancommunication receiver used to enable the satellite to exchange datawith the linked up satellite, or ground station. The monolithicstructure is configured to dispose the optic axes between the telescopeand laser acquisition and communication lasers and the optic axesbetween the telescope and the acquisition and communication receivers insubstantially a common plane. With such an arrangement, the structuralrigidity and hence optical integrity of the monolithic structure isimproved.

[0005] In accordance with an additional feature of the invention, asingle detector is provided for both the acquisition mode and asubsequent tracking mode. More particularly, the laser communicationsystem includes a tracking laser transmitter and a tracking receiverused to enable the satellites to track each other during the trackingmode and thereby maintain the link up with the other satellite, orground station, after the above described acquisition mode. Thesatellites communicate with one another during the tracking mode. In apreferred embodiment of the invention, a single charge coupled device(CCD) is used.

[0006] In accordance with still another feature of the invention, acollimating/beam shaping module is provided having affixed thereto apair of submodular units. More particularly, as noted above, the laser'slight passes through the monolithic structure solely as collimatedlight. In passing between a laser in the system and the monolithicstructure, the laser's light beam must shaped and collimated. The firstsubmodular unit includes the laser and a properly aligned beam shapinglens. The second submodular unit includes a mounted collimating lens.The first and second submodular units are aligned with each other andthen affixed to each other to provide the collimating/beam shapingmodule. Next, the collimating/beam shaping module is affixed to themonolithic structure. With such arrangement and method proper accuratealignment of the mounted laser, beam shaping lenses and collimating lensis facilitated.

[0007] In accordance with still another feature of the invention, afilter is provided on a surface of the second submodular unit. Thefilter protrudes beyond the second submodular unit and is provided witha surface adapted to interface, and be affixed to, a surface portion ofthe monolithic structure.

BRIEF DESCRIPTION OF THE DRAWING

[0008] For a more complete understanding of the concepts of theinvention, as well as the invention itself, reference is now made to thefollowing drawings, in which:

[0009]FIG. 1 is a sketch of a satellite communication system wherein apair of satellites communicate with each other using an inter-satellitecross link routing process, each one of such satellite carrying a lasercommunication system according to the invention;

[0010]FIG. 2 is a plan view of the laser communication system used inthe satellite communication system of FIG. 1;

[0011]FIG. 3 is a cross section, elevation view of a laser transmittermodule used in the laser communication system of FIG. 2;

[0012]FIG. 4 is a cross section view of the laser transmitter module ofFIG. 3, such cross section being taken along line 4-4 in FIG. 3;

[0013]FIG. 5 is a cross sectional elevation view of a submodular unitsof the module of FIG. 3, such cross section being taken along line 5-5in FIG. 4;

[0014]FIG. 6 is a diagram useful in understanding a tracking system usedin the laser communication system of FIG. 1;

[0015]FIG. 7 is a block diagram of a control system used in the trackingsystem of FIG. 6; and

[0016]FIG. 8 is a diagram useful in understanding the tracking system ofFIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Referring now to FIG. 1, a laser satellite communication system10 is shown. Here, a ground station 12 transmits information to anorbiting satellite 14, the satellite 14 receives the information andrelays it, via a laser communication system 16, to be described indetail hereinafter, on board the satellite 14, to a second orbitingsatellite 18 in an inter-satellite cross link in the routing process.The second orbiting satellite 18, also carries a laser communicationsystem, not shown, such as the system 16 carried by satellite 14, andthen transmits the information to a second ground station 20, as shown.Here the information is transmitted between the satellites 14, 18 andthe ground stations 12, 20 using radio frequency signals. It should beunderstood however that laser (i.e., light) energy signals may also beused. Further, while only two satellites 14, 18 have been shown, manymore satellites may be cross linked; either in a low earth orbitconstellation (LEO) or in a geosynchronous orbit (GEO) constellation.

[0018] Referring now to FIG. 2, the laser communication system 16disposed in each one of the satellites 14, 18 includes: a pair 22 ofacquisition laser transmitters 22 a, 22 b; a pair 24 of tracking lasertransmitters 24 a, 24 b; a pair 26 of communication laser transmitters26 a, 26 b; an acquisition/tracking detector/receiver 28; acommunication detector/receiver 30; and a beam director 33, here made upof a fine steering mirror 31 and a telescope 32, all optically coupledtogether in a manner to be described in detail hereinafter by amonolithic optical structure 34. suffice it to say here, however, thatthe monolithic optical structure 34 includes a plurality of active andpassive optical elements for interfacing between the beam director 33and the laser transmitters 22 a, 22 b, 24 a, 24 b, 26 a, 26 b and thedetector/receivers 28, 30. Laser energy is directed between the lasertransmitters 22 a, 22 b, 24 a, 24 b, 26 a, 26 b and the beam director 33and between the beam director 33 and the detector/receivers 28, 30 bythe active and passive optical elements. The laser energy passes throughthe monolithic optical structure 34 solely as collimated light. Themonolithic optical structure 34 is configured so that all optic axesbetween the beam director 33, the laser transmitters 22 a, 22 b, 24 a,24 b, 26 a, 26 b and the detector/receivers 28, 30 are disposed insubstantially a common plane, here the X-Y plane. The acquisition lasertransmitters 22 a, 22 b and an acquisition/tracking detector/receiver 28are used to enable the satellite to link up with another satellite, orground station, during an acquisition mode. The tracking lasertransmitters 24 a, 24 b are used to maintain track with the othersatellite, or ground station during a subsequent tracking mode. Thecommunication laser transmitters 26 a, 26 b and communicationdetector/receiver 30 are adapted to enable the satellite to exchangedata with the linked up satellite, or ground station during the trackingmode. As noted above, the monolithic optical structure 34 is configuredto dispose the optic axis between the beam director 33 and laseracquisition, tracking and communication laser transmitters 22 a, 22 b,24 a, 24 b, 26 a, 26 b and the optic axes between the telescope 32 andthe acquisition and communication detector/receivers 28, 30 insubstantially a common plane, here the X-Y plane.

[0019] As noted above, the laser transmitters 22, 24, 26 anddetector/receivers 28, 30 include: tracking laser transmitter 24 forenabling the linked up satellite, or ground station to track thesatellite during a tracking mode. A single acquisition/track, here (A/T)detector 28, is used by the satellite during both the acquisition modeand the subsequent tracking mode. As will be described in detailhereinafter, the single detector 28 uses a charge coupled device (CCD).

[0020] The laser communication system 16 includes for each one of thelaser transmitters 22 a, 22 b, 24 a, 24 b, 26 a, 26 b a collimating/beamshaping module 38. Each one of the module 38 will be described in detailhereafter in connection with FIGS. 3, 4 and 5. Suffice it to say here,however, that the module 38 includes a pair of submodular units 40, 42.A first one of the submodular units, here modular unit 40 includes oneof the transmitting lasers 22 a, 22 b, 24 a, 24 b, 26 a, 26 b,respectively, as shown, and a beam shaping lens 41, the second one ofthe submodular units 42 having a collimating lens 44, as shown. Thecollimating lens 44 is, here, a molded aspheric collimating lens. Thefirst and second submodular units 40, 42 are aligned with, and affixedto, each other to provide the collimating/beam shaping module 38. Thecollimating/beam shaping module 38 is then affixed to the monolithicoptical structure 34. A bandpass filters 150, 152, 154, 156, 158 and 160are disposed on an surface of the second submodular units 42, as shown.The bandpass filter 150-160 protrudes beyond the second submodular unit42 and is provided with a surface adapted to interface, and be affixedto, a surface portion of the monolithic optical structure 34.

[0021] More particularly, the monolithic optical structure 34 includes aplurality of glass cubes and planar thin films bonded together anddisposed to pass such light signals therethrough as only substantiallycollimated light. The pair of acquisition laser transmitters 22 includea primary acquisition laser transmitter 22 b and a redundant acquisitionlaser transmitter 22 a. Here, both such acquisition laser transmitters22 a, 22 b transmit light at a predetermined wavelength, here 810 nm,but which can operate from 804 to 816 nm over temperature. The primaryacquisition laser transmitter 22 b here transmits light with, here,vertical polarization; while the redundant acquisition laser transmitter22 a transmits such light with, here, horizontal polarization. The pairof tracking laser transmitters 24 include a primary tracking lasertransmitter 24 b and a redundant tracking laser transmitter 24 a. Here,both such tracking lasers transmitters 24 a, 24 b transmit light at apredetermined wavelength, here also 810 nm. It is again noted thatbecause the acquisition and tracking laser transmitters 22, 24 use thesame wavelength, the laser transmitters 22, 24 operate sequentiallyrather than simultaneously. Here both the primary and redundant trackinglaser transmitters 24 a, 24 b transmit light with the same, herehorizontal polarization. The a pair of communication laser transmitters26 include a primary laser transmitter 26 b and a redundantcommunication laser transmitter 26 a. Here, both such communicationlasers transmitters 26 a, 26 b transmit light at a predeterminedwavelength, here a longer wavelength of 860 nm. Here, both the primaryand redundant communication laser transmitters 26 a, 26 b transmit lightwith the same, here horizontal polarization. The acquisition/trackingdetector/receiver 28 is adapted to operate with laser energy of, here,780 nm while the communication detector/receiver 30 is here adapted tooperate with laser energy of 830 nm. Here, the telescope 32 is aCassegrainian telescope of any conventional design.

[0022] As noted above, the monolithic optical structure 38 includes thinfilms. One type of thin film used is; here a liquid crystal polarizationrotator. As discussed in my paper entitled “Technologies and techniquesfor lasercom terminal size, weight, and cost reduction” Free-Space LaserCommunication Technology II, Proc. SPIE, VOL. 1218, pp. 62-69, 1990, myco-authored paper entitled “Liquid crystals for laser applications” byChinh Tan and myself, Robert T. Carlson, published January 1991 inProceedings of the SPIE, #1866 as well as in my co-authored paperentitled “An advanced lasercom terminal for intersatellite crosslinks”by myself, Robert T. Carlson, Voula C. Georgeopolous, and Jerold L.Jaeger, published Mar. 2, 1994 in The Proceedings of the 15thInternational Communications Satellite Systems Conference, March, 1994,the material in all such papers being incorporated herein by reference,by applying a proper voltage on the liquid crystal film, here nematicliquid crystal, light of different polarizations may be directeddifferently. Here, for example, thin films of nematic liquid crystal atlow voltage provide no phase retardation to the light passingtherethrough with the result that the polarization of such light remainsunchanged. At maximum voltage, however, the polarization of lightentering the nematic liquid crystal is rotated 90 degrees so thatvertically polarized light entering the crystal leaves the crystal ashorizontally polarized light, on the one hand, and horizontallypolarized light entering the crystal leaves the crystal as verticallypolarized light, on the other hand. Thus, by placing a polarizationbeamsplitter in the path of the light leaving the liquid crystalrotator, vertically polarized light is, for example, reflected by thebeamsplitter to pass along one direction, horizontally polarized lightis transmitted through the beamsplitter to pass along another direction.Thus, light may be directed to pass in one of the two directions,selectively, in accordance with the electrical voltage (i.e., controlsignals) applied to the liquid crystal rotator. Thus, here, as will bedescribed in detail hereinafter, the thin films are responsive toelectrical control signals to direct the light signals from either thefirst primary laser transmitter 22 b, 24 b, 26 b or the redundant lasertransmitter 22 a, 24 a, 26 a through the monolithic optical structure 34to the beam director 33, selectively, in accordance with the controlsignals. Such is the case for each of the pair of laser transmitters 22,24, 26; i.e., the pair of acquisition laser transmitters 22 (i.e., theprimary acquisition laser transmitter 22 b and the redundant acquisitionlaser transmitter 22 a), the pair of tracking laser transmitters 24(i.e., the primary tracking laser transmitter 24 b and the redundanttracking laser transmitter 24 a) and the pair of communication lasertransmitters 26 (i.e., the primary laser transmitter 26 b and theredundant communication laser transmitter 26 a). Additionally, a liquidcrystal polarization rotator thin film is responsive to electricalcontrol signals to attenuate light from the sun, or signals from anothersatellite which may saturate either one of the detector/receivers 28,30. Here, the control signals are developed from the detector/receiversand provide a feedback signal to the thin film, as will be describedhereinafter.

[0023] Referring now to the details of the monolithic optical structure34, it should first be noted that the structure includes a plurality of,here twelve glass cubes, here one-half inch glass cubes, which provide:five polarization beamsplitters (PBS) 50, 52, 54, 56, 58, two dichroicbeamsplitters (DBS) 60, 62, four folding mirrors 64, 66, 68, 70, and aspacer 72. Also included are a plurality of, here nine active thin film,nematic liquid crystal (LC) polarization rotators: two X-Y planealignment liquid crystal polarization rotators (LC X-Y PLANE ALIGN) 106,108 each responsive to control signals on lines 110, 112, respectively,two Y-Z plane alignment liquid crystal polarization rotators (Y-Z PLANEALIGN) 114, 116, each responsive to control signals on lines 118, 120respectively, and five 0/90 degree phase retardation liquid crystalpolarization rotators (LC POL ROT) 122, 124, 126, 128 and 130, eachresponsive to control signals on lines 132, 134, 136, 138, 140, Alsoincluded are eight passive optical wedges for course alignment (ALIGN)74, 76, 78, 80, 82, 84, 86, and 88. The course alignment wedges 74, 76,78, 80, 82, 84, 86, and 88 are, here, fused silica spacers polished tothe required wedge angle to cause course alignment in the X-Y and Y-Zplanes. Precision alignment is subsequently accomplished with the liquidcrystal X-Y plane and Y-Z plane alignment devices 106, 108, 114, and116. (It should be noted that the course alignment wedges 74-88 areadapted to deviate the beam, or optic axis, and the X-Y plane and Y-Zplane alignment wedges 106, 108, 114, 106 are adapted to deviate thebeam, or optic axis, one milliradian, or less. Therefore, whiledeviations of up to about ten milliradians are possible, all optic axesbetween the laser transmitters 24, 26, 28 and the beam director 33 andbetween the beam director 33 and the detector/receivers 28, 30 aresubstantially in a common plane. To put it another way, all opticalblocks, thin films and other optical elements of the monolithic opticalstructure 34 are in a common plane, here the X-Y plane.) Also includedis a plurality of, here nineteen, passive thin film devices: three halfwave plates 142, 144, 146, one quarter wave plate 148, eight bandpassfilters 150, 152, 154, 156, 158, 160, 162, 164, and three polarizationfilters 166, 168, 170 used to purify the desired linear polarization,two spacers 172, 174, to provide proper alignment of the blocks, and twoabsorptive neutral filters (ND) 173, 175. A heat sink 178 is mounted, asshown. The elements 50-58, 60-62, 64-70, 72, 74-88, 106-108, 114-116,122-130, 142-146, 148, 150-164, 166-170, 172-175 are permanently bondedtogether with any suitable optical cement, to form a monolithic opticalstructure 32, as shown; all such elements being disposed in a commonplane, here the X-Y plane, as shown. Fused silica is used throughoutbecause of its superb radiation resistance and thermal stability. Themonolithic optical structure 34 is here, less than 6 cubic inches (sixinches along the X axis, 2.5 inches along the Y axis and 0.5 inchesalong the Z axis) and weighs less than 1 pound. Because optical pathsare short, all light passing through the structure 34 is substantiallycollimated light thereby eliminating relay, or focusing opticalassemblies and their concomitant alignment requirements. Further, a pairof lenses, beam shaping lens 41, collimating lens 44 are mounted withina single module 38, along with their associated laser transmitter, in amanner to be described in detail hereinafter in connection with FIGS. 3,4, and 5. The lenses 41, 44 are used for beam shaping and collimating,respectively, the light transmitted to the monolithic optical structure34 by the laser transmitters 22, 24, 26. Lenses 45, 47 are used forfocusing the collimated light exiting the monolithic optical structure34 to the detector/receivers 28, 30, respectively, as shown.

[0024] As noted above, light transmitted by either one of thecommunication laser transmitters 26 a, 26 b (i.e., entering themonolithic optical structure 34) is horizontally polarized. Likewise,light transmitted by either one of the tracking laser transmitters 24 a,24 b is horizontally polarized. However, the wavelength of the lighttransmitted by the communications laser transmitters 26 a, 26 b and bythe tracking laser transmitters 24 a, 24 b are different. The wavelengthof the light transmitted by either one of the tracking lasertransmitters 24 a, 24 b is shorter than the light transmitted by eitherone of the communication laser transmitters 26 a, 26 b. Here, thewavelength of each one of the communication laser transmitters 26 a, 26b is 860 nm and the wavelength of each one of the tracking lasertransmitters 24 a, 24 b is 810 nm.

[0025] The horizontally polarized light from the primary communicationlaser transmitter 26 b passes through the bandpass filter 158 to halfwave plate 146 for conversion to vertical polarization. The verticallypolarized light passes through the course alignment wedge 82 and is thenreflected by the folding mirror 68 to a polarization beamsplitter 58.The polarization beamsplitter 58 reflects the vertically polarized lightto the liquid crystal polarization rotator 128. In the low voltagestate, the liquid crystal polarization rotator 128 passes the verticallypolarized to the Y-axis and X-axis alignment liquid crystal devices 116,108, to a dichroic beamsplitter 60, here designed for verticallypolarized light. Here, the dichroic beamsplitter is designed to reflectlight having a wavelength of, here 860 nm and transmit light having awavelength of, here 810 nm. Thus, the dichroic beamsplitter 60 isdesigned to reflect the higher, or longest of these two wavelengths andto transmit light having the shortest of these two wavelengths.Therefore, the dichroic beamsplitter 60 reflects the longer wavelength,vertically polarized light from the primary communication lasertransmitter 26 b through spacer 174 to a half wave plate 142 forconversion from vertical polarization to horizontal polarization. Thehorizontally polarized light passes through a polarization filter 166 toa polarization beamsplitter 54. The polarization beamsplitter 54transmits the horizontally polarized light to a quarter wave plate 148for conversion to right hand circular polarization. The right handcircularly polarized light is directed by the beam director 33 to areceiver external to the satellite.

[0026] The horizontally polarized light from the redundant communicationlaser transmitter 26 a passes through the bandpass filter 160 and coursealignment wedge 84 to a polarization beamsplitter 58. (Unlike the lightfrom the primary laser transmitter, the light from the redundantcommunication laser transmitter does not pass through a half wave plateto the polarization beamsplitter; thus, the horizontally polarized lightof the redundant communication transmitting laser remains horizontallypolarized). The polarization beamsplitter 58 transmits the horizontallypolarized light to the liquid crystal polarization rotator 128. In ahigh voltage state, the liquid crystal polarization rotator 128 rotatesthe horizontally polarized light 90 degrees and thereby converts thehorizontally polarized light to vertical polarization. The verticallypolarized light passes through Y-axis and X-axis alignment liquidcrystal devices 116, 118 to the dichroic beamsplitter 60, here, as notedabove designed for vertically polarized light, to reflect suchvertically polarized light if such light has the higher of twowavelengths. Here, as noted above, the wavelength is 860 nm. Thereflected, vertically polarized light passes through spacer 174 to ahalf wave plate 142 for conversion from vertical polarization tohorizontal polarization. The horizontally polarized light passes througha polarization filter 166 to a polarization beamsplitter 54. Thepolarization beamsplitter transmits the horizontally polarized light toa quarter wave plate 148 for conversion to right hand circularpolarization. The right hand circularly polarized light is directed bythe beam director 33 to a receiver external to the satellite.

[0027] The horizontally polarized light from the primary tracking lasertransmitter 24 b passes through a bandpass filter 154 to a half waveplate 144 for conversion to vertical polarization. The verticallypolarized light passes through course alignment wedge 78 and is thenreflected by the folding mirror 66 to the polarization beamsplitter 56.The polarization beamsplitter 56 reflects the vertically polarized lightto the liquid crystal polarization rotator 126. In the low voltagestate, the liquid crystal polarization rotator remains verticallypolarized through the Y-Z plane and X-Y plane alignment liquid crystaldevices 114, 106 to another polarization beamsplitter 52. Thepolarization beamsplitter reflects the vertically polarized light to aliquid crystal polarization rotator 124. The voltage on the liquidcrystal polarization rotator is low so that the vertically polarizedlight remains vertically polarized. The vertically polarized lightpasses through the spacer 172 to the dichroic beamsplitter 60. Thedichroic beamsplitter 60 transmits the 810 nm wavelength, verticallypolarized light through spacer 174 to the half wave plate 142. The halfwave plate 142 converts the vertically polarized light to horizontallypolarized light. The horizontally polarized light passes through thepolarization filter 166 to a polarization beamsplitter 54. Thepolarization beamsplitter 54 transmits the horizontally polarized lightto a quarter wave plate 148 for conversion to right hand circularpolarization. The right hand circularly polarized light is directed bythe beam director 33 to a receiver external to the satellite.

[0028] The horizontally polarized light from the redundant trackinglaser transmitter 24 a passes through a bandpass filter 156 and coursealignment wedge 80 to a polarization beamsplitter 56. (Unlike the lightfrom the primary tracking laser transmitter, the light from theredundant tracking laser transmitter does not pass through a half waveplate to the polarization beamsplitter; thus, the horizontally polarizedlight of the redundant tracking transmitting laser remains horizontallypolarized). The polarization beamsplitter 56 transmits the horizontallypolarized light to the liquid crystal polarization rotator 126. In a lowvoltage state, the horizontally polarized light remains horizontallypolarized as it passes through the liquid crystal polarization rotator126. The vertically polarized light passes through the Y-Z plane and X-Yplane alignment liquid crystal devices 114, 106 to a second polarizationbeamsplitter 52. The polarization beamsplitter 52 reflects thevertically polarized light to a second liquid crystal polarizationrotator 124. In a low voltage state, the vertically polarized lightremains vertically polarized. The vertically polarized light is passesthrough spacer 172 to a dichroic beamsplitter 60. Here, the redundanttracking laser transmitter also has the shorter, 810 nm wavelength. Thedichroic beamsplitter 60 transmits the vertically polarized lightthrough spacer 174 a half wave plate 142 for conversion from verticalpolarization to horizontal polarization. The horizontally polarizedlight passes through a polarization filter 166 to a polarizationbeamsplitter 54. The polarization beamsplitter 54 transmits thehorizontally polarized light to a quarter wave plate 148 for conversionto right hand circular polarization. The right hand circularly polarizedlight is directed by the beam director 33 to a receiver external to thesatellite.

[0029] The primary and secondary acquisition laser transmitters 22 b, 22a transmit light at 810 nm; i.e., the shorter wavelength. Further, theprimary acquisition laser transmitter 22 b transmits light with verticalpolarization; while the redundant laser transmitter 22 a transmits lightwith horizontal polarization.

[0030] Thus, the vertically polarized light transmitted by the primaryacquisition laser transmitter 22 b passes through bandpass filter 152and alignment wedge 76 and is then reflected by folding mirror 64 topolarization beamsplitter 50. The vertically polarized light isreflected by the polarization beamsplitter 50 to a liquid crystalpolarization rotator 122. In a high voltage state, the liquid crystalpolarization rotator 122 converts the vertically polarized light tohorizontal polarization. The horizontally polarized light is transmittedby a second polarization beamsplitter 52 to a second liquid crystalpolarization rotator 124. In a low voltage state, the horizontallypolarized light remains horizontally polarized as it passes through theliquid crystal polarization rotator 124. The shorter wavelength,horizontally polarized light passes through spacer 172 to dichroicbeamsplitter 60 and is then transmitted by the dichroic beamsplitter 60,through spacer 174, to a half wave plate 142. The half wave plate 142converts the horizontally polarized light to vertical polarization. Thevertically polarized light is transmitted through polarization filter166 to polarization beamsplitter 54. The polarization beamsplitter 54transmits the vertically polarized light, to quarter wave plate 148 forconversion to right hand circular polarization. The right handcircularly polarized light is directed by the beam director 33 to areceiver external to the satellite.

[0031] The horizontally polarized light transmitted by the redundantacquisition laser transmitter 22 a is transmitted through a bandpassfilter 150 and alignment wedge 74 to a polarization beamsplitter 50. Thehorizontally polarized light is transmitted by the polarizationbeamsplitter 50 to a liquid crystal polarization rotator 122. In a lowvoltage state, horizontally polarized light remains horizontallypolarized as it passes through the liquid crystal polarization rotator122. The horizontally polarized light is transmitted by polarizationbeamsplitter to a second liquid crystal polarization rotator 124. In alow voltage state, the horizontally polarized light remains horizontallypolarized as it passes through the liquid crystal polarization rotator124. The shorter wavelength, horizontally polarized light is transmittedthrough spacer 172 to a dichroic beamsplitter 60. The light istransmitted through the dichroic beamsplitter 60, through spacer 174, toa half wave plate 142. The half wave plate 142 converts the horizontallypolarized light to vertical polarization. The vertically polarized lightis transmitted through the polarization filter 166 and polarizationbeamsplitter 54 to quarter wave plate 148 for conversion to right handcircular polarization. The right hand circularly polarized light isdirected by the beam director 33 to a receiver external to thesatellite.

[0032] Considering now received light, during he acquisition andtracking modes, left hand circularly polarized light received from asource external to the satellite is directed by the beam director 33 tothe quarter wave plate 148 for conversion to vertically polarized light.The vertically polarized light is reflected by the polarizationbeamsplitter 54, through polarization filter 168 (to purify the verticalpolarization), to liquid crystal polarization rotator 130. It should benoted that the liquid crystal polarization rotator 130 in the path ofthe received light is used to attenuate light from the sun, or incomingsignals from another satellite. Thus, if either one of the detectors 28or 30 tend to saturate, a feedback signal is developed by a processor131 fed by the outputs of the detectors 28, 30 to increase the voltageon the liquid crystal polarization rotator 130 via line 140. This, tendsto rotate the polarization, which causes attenuation by the polarizationfilter 170. The light then passes to the dichroic beamsplitter 62.During the acquisition and tracking modes, the light used has awavelength of here, 780 nm. Light used for communications here has awavelength of 830 nm. Thus, the acquisition and tracking light have theshorter of the two wavelengths and is transmitted by the dichroicbeamsplitter 62, through course alignment wedge 86 and bandpass filter162, to acquisition/tracking detector 28.

[0033] Likewise, left hand circular polarized light received by thesatellite with communication information is also converted to verticallypolarized light by the quarter wave plate 148 and is reflected by thepolarization beamsplitter 54, through the polarization filter 168, tothe liquid crystal polarization rotator 130. Here again the controlsignal on line 140 from processor 131 provides a feedback system toattenuate light from the sun, or from another satellite, which tends tosaturate either one of the detectors 28, 30 in the manner describedabove. The light then passes to the dichroic beamsplitter 62. Becausethe light used for communications here has a wavelength of 830 nm, i.e.,the longer wavelength as compared to the wavelength of the lightreceived for acquisition and tracking, the 830 nm wavelength, verticallypolarized light is reflected by the dichroic beamsplitter 62, throughspacer 72, to folding mirror 70. The light is reflected by the foldingmirror, through course alignment wedge 88 and bandpass filter 164, tocommunication detector 30.

[0034] The bandpass filters 150-164 are centered at the wavelength ofthe laser light to be passed by such filters and are used in both thetransmit and receive channels and to provide channel-to-channel andtransmit-receive isolation (i.e., reject all other, unwanted, opticalwavelengths). As noted from the above description, the polarizationbeamsplitters 50, 56, and 58 are used as primary and redundant channelcombiners. Dichroic beamsplitters 60 and 62 are used for wavelengthdivision multiplexing on both the transmit and the receive side of themonolithic optical structure. Electro-optic liquid crystal polarizationdevices, 122, 124, 126, 128 and 130 or phase retarders, are used aspolarization rotators for redundancy implementation, and as a strongsignal attenuator in the receive path. Electro-optic liquid crystalwedges 106, 108, 114 and 116 are used as precision X-Y plane, Y-Z planebeam deflectors for alignment. More particularly, with the liquidcrystal wedges 106, 108, 114, 116 the nematic liquid crystals are in awedge shaped structure (sandwitched between a pair of plates havingplanar outer surfaces) so that when a voltage is applied to the liquidcrystal the corresponding change in index of refraction causes adeflection in the beam of light passing through the wedge. The X-Y planealignment wedges 106, 108 deflects the beam in the X-Y plane, and theY-Z plane alignment wedges 114, 116 deflects the beam in the Y-Z plane.The degree of deflection is controlled by the level of the voltage fedto the wedges 106, 108, 114, 116 established during an initial alignmentprocess and which are maintained during normal operation.

[0035] The polarization beamsplitters 50, 52, 54, 56, 58 include of apair of right angle fused silica prisms cemented together along thehypotenuse, with an embedded multilayer dielectric thin filmbeamsplitter coating. The dichroic beamsplitters 60, 62 are made of twocemented right-angle prisms with the hypotenuse of one prism coated. Theincident light is perpendicular to one face of the cube and thetransmitted light exits through the opposite face. The reflected lightmakes a 90° angle with the incident light and exits through a side face.The dichroic beamsplitters 60, 62, are designed to be selective fors-polarized light. The dichroic beamsplitters 60, 62 are, as discussedabove, used for multiplexing the acquisition/tracking and communicationchannels on the transmit side and demultiplexing theacquisition/tracking and communication channels on the receive side ofthe terminal. Therefore, there are two dichroic beamsplitters 60, 62, asdiscussed. A 810/860 nm dichroic beamsplitter 60 for the lasertransmitters 22, 24, 26, and an 780/830 nm dichroic beamsplitter 62 forthe detector/receivers 28, 30, as discussed above.

[0036] As noted above, liquid crystal polarization rotators 122, 124,126, 128, 130 are used as voltage-controlled electro-optic polarizationrotators. Retarders, also called waveplates, are optical devices thatdivide a light wave into two orthogonal vector components and produce aphase shift between these two components. The components recombine onleaving the device to give a light wave generally of a differentpolarization form, as discussed above. The liquid crystal polarizationrotators, or retarders, rotate p-polarized light into s-polarized andvice versa, for vernier polarization rotation, redundancy switching,path-switching (as for rotators 124, 126, 128, and as avoltage-controlled receive intensity attenuator (as for rotator 130).The nematic liquid crystal cells used are polarization rotators with anelectrically adjustable retardance (phase shift). The retardance can beadjusted by applying a 2 kHz square wave ac voltage to the liquidcrystal. The retardance decreases as the amplitude of the appliedvoltage increases. An amplitude of only a couple volts is required.

[0037] Referring now to FIG. 3, an exemplary one of the lasertransmitter module 38 is shown. As noted above, the module 38 includes apair of submodular units 40, 42. Submodular unit 40 being shown in FIGS.4, 5, and 5A. Each one of the submodular units 40 includes acorresponding one of the laser transmitters 22 a, 22 b, 24 a, 24 b, 26a, 26 b, here laser transmitter 22 a being shown in FIGS. 3, 4 and 5A, abeam shaping lens 41 and an optical window 39, as shown. Submodular unit42 has the collimating lens 44, as shown. The first and secondsubmodular units 40, 42 are aligned with, and affixed to, each other toprovide the collimating/beam shaping module 38. The collimating/beamshaping module 38 is then affixed, here bonded with optical cement, tothe monolithic optical structure 34. A corresponding one of the bandpassfilters 150-160, here bandpass filter 150 is disposed on an surface ofthe second submodular unit 42. The filter 150 protrudes beyond thesecond submodular unit 42 and is provided with a surface adapted tointerface, and be affixed to, a surface portion of the monolithicoptical structure 34, here to course alignment wedge 74.

[0038] The laser transmitter module 38 is adapted to provide a beam ofcollimated light to the diffraction limit. Here, the module 38 is aboutone cubic inch in volume. The submodular unit 40, includes a lasersubmount 200 for securing the laser transmitter 22 a, here asemiconductor laser chip. (The laser transmitters are diode lasers herewith 3-5 watts for acquisition and 150 milli-Watts for communicationsand tracking). The multi-Watt acquisition laser transmitters 22 a, 22 bare broad area devices that are not diffraction limited, but capable offlooding a 1-2 milliradian acquisition field of view. A microlens 41 isbonded very close to the laser emitting facet, here 1 to 4 mils, by anepoxy 202, as shown in FIG. 5. The microlens 41 is aligned with thelaser 22 a using multi-axis micro-positioning translation stages. Themicrolens 41 is a aspheric rod microlens for anamorphic correction ofthe laser beam. A thermistor 210 is bonded to the upper surface of thelaser submount 200, as shown. The laser submount 200 is mounted on athermoelectric cooler 214, as shown. The bottom surface of the cooler214 is disposed on a heat transfer device 216, here a molybdenum heattransfer slug, as shown. An alumina substrate 218, here 25 mils thick,is provided to support hybrid electronic driver circuitry 220 for thelaser transmitter 22 a. More particularly, in the case of thecommunication laser transmitter, information signals received by thesatellite from the ground station, or the other satellite in the crosslink are fed to the module via an input line 224. (In the case of theacquisition and tracking laser transmitters 22, 24 the signals timemultiplex the operation of these laser transmitters 22, 24 to enablesuccessive, non-concurrent operation. Such signals are provided by aconventional control circuit, here included in the hybrid drivercircuitry 220 to modulate the laser in accordance with signals from theground station 12 or satellite 18, FIG. 1. More particular, theinformation line 224 passes through a feedthrough 226 provided in thelower section 228 of a two piece hermetically sealed package 230, heremade of Kovar material (Ni—Co—Fe), as shown. The alumina substrate 218also has disposed on the upper surface thereof a laser energy detector230, as shown. The detector 230 is disposed under laser submount 200 toreceive a small fraction of the laser energy from the rear facet of thelaser transmitter; the dominant portion of the laser produced energypassing upwardly, in FIG. 3, through the beam forming microlens 41,through a sapphire window 39 mounted to submodular units 40. The twosections 228, 232 of the Kovar package, or submodular units 40 arebonded together to form the submodular units 40, here by solder. Thebottom surface of the alumina substrate 218 is also disposed on the heattransfer device 216, as shown. One method for installing the sapphirewindow 39 is with a borosilicate glass preform (annulus), not shown,having a moderate melting temperature that permits installation of thesapphire window 41 without degrading its optical quality. The fusiblematerial, not shown, should melt at a temperature far below thesoftening temperature of the window and should match the thermalexpansion coefficient of both the sapphire window 39 and the submodularunits 40 package material. This method permits sealing of the sapphirewindow 39 into the package 40 without introducing mounting wavefrontdistortion into the window 39.

[0039] Preferably, the laser beam produced by the laser transmitter 22 ashould exit the laser transmitter 22 a through the beam shapingmicrolens 41 with less than 1 milliradian deviation from theperpendicularity with respect to the mechanical axis of the module 38.

[0040] The laser transmitter 22 a is be cooled by the cooler 214 to pullits nominal as-procured wavelength down to its required wavelength.This, in turn, requires a hermetically sealed package 40 filled with aninert gas in order to prevent condensation on the laser facet. Thethermistor 210 measures the temperature of the laser transmitter 22 a anprovides a feedback control signal to the cooler 214, to provide propertemperature control for the laser transmitter 22 a. The Kovar materialpreferably used for the package 40 has a thermal expansion coefficient(5.3×10⁻⁶/degree C) to match the thermal expansion coefficient of theborosilicate glass and also to match the thermal expansion coefficientof the alumina substrate 218 (6.7×10⁻⁶/degree C). It also matches thethermal coefficient of expansion of the sapphire window 39. Here, theKovar package 40 has a molybdenum heat sink slug, as noted above, toprovide a low thermal resistance to heat sink 219. Molybdenum ispreferred because it has the same thermal expansion coefficient as theKovar package 40, but significantly better thermal conductivity (140versus 17 W/m-degrees C) to efficiently couple the heat out of thethermal cooler 204 and into the heat sink 219 via the heat transferdevice 216. An alternative to the use of the molybdenum thermal slug isa metal matrix packaging material, such as SILVAR material developed byTexas Instruments, machined or stamped, and attached with epoxy, solderor braze, may be used. Such material permits the use of fused glassseals, such material also has a high thermal conductivity comparable tomolybdenum (157 versus 140 W/m-degrees C), thereby eliminating the needfor a separate thermal slug for the package base 216. As noted above,the thermistor 210 and thermoelectric cooler 214 are provided for lasertransmitter 22 a operating wavelength and output power stabilization bytemperature control. Also provided is a hybrid laser driver circuit 220,here adapted to provide 200-300 milliamps at modulation rates of up to155 Mbps. The base 216 of the submodular units 40 conducts heat from thelower, warm side of the thermoelectric cooler 214′ and the hybrid drivercircuitry 220 to heat sink 219 via the base, or heat transfer device216. A flexible thermal interface 217 having a high thermal conductivityis provided between the heat transfer device 216 and the heat sink 219,as shown, to accommodate differences in thermal expansion between theheat transfer device 216 and the heat sink 219). The flexible interface217 preferable is a thermal tape with a binder, a silicone-based thermalgrease, or a conductive adhesive with the appropriate thermalproperties.

[0041] The laser transmitter metal package, or submodular units 40, isbonded to submodular units 42. More particularly, the submodular units40 is bonded onto one end of a collimating lens sleeve 240. The lenssleeve 240 provides a holder for the collimating lens 44 and as amechanical interface for submodular unit 40. The use of two submodularunits 40, 42 for the laser transmitter module 38 splits the lasercollimating lens 44 optics and the laser transmitter with its microlensbeam shaping lens 41 on its facet optics and thereby provides for twoseparately aligned units (i.e, submodular unit 40 and submodular unit42). The use of two separate aligned units, or submodular units 40, 42relaxes the collimating lens 44 alignment tolerances, as compared to theuse of a single module having both the collimating lens 44 and the lasertransmitter with its microlens 39 on its facet, because the f/number ofthe beam into the collimating lens 44 is, typically on the order off/2.5 rather than the f/1.0 beam that exits the laser transmitter facet.

[0042] Preferably, the lens sleeve 240 is an opaque ceramic, a compositematerial or a metal matrix with low thermal coefficient of expansion sothat thermal excursions will not induce a radial tensile strain into thecollimating lens 44 or the bandpass filter 150 (FIG. 2) bonded to theceramic sleeve 240. Material for the sleeve 240 preferably are an Invarmaterial or graphite composite (less than or equal to 0.2×10⁻⁶/degreesC) or carbide-machinable ceramic (9.4×10⁻⁶/degrees C), such as Macormaterial from Corning, depending on the desired expected thermalvariation.

[0043] Referring now to FIG. 6, the other satellite 18 (FIG. 1) linkedwith the satellite 16, is shown at a first instant in time t1 and at asomewhat later instant in time t2. The laser energy transmitted by theacquisition/tracking lasers, not shown, in satellite 18, indicated byarrow 90, is directed by the fine steering mirror 31 to theacquisition/tracking detector 28 of satellite 16, through the monolithicoptical structure 34. (It is noted that the monolithic optical structure34 is only diagrammatically represented in FIG. 6). The light receivedby satellite 16 from satellite 18 at time t1 is directed to the detector28. Here, detector 28 uses a 15. charge coupled device having an arrayof rows and columns of detector pixels, as shown, in FIG. 8. As shown bythe “o” in FIG. 8, the received light is here shown focused to a pixelhaving an X, Y coordinate of Xn, Ym. The center, or boresight, or opticaxis is indicated by the “x” in FIG. 8. Because of the relative motionbetween satellite 16 and satellite 18, here indicated by arrow 92 inFIG. 6, it is necessary that the beam of laser energy transmitted bysatellite 16, indicated by arrow 94, “lead”, or point ahead of, thelight (arrow 90) transmitted by the satellite 18 by a lead angle, L. Acontrol system 96 (FIG. 6) responds to the pixel in detector 28 (FIG.8), here pixel Xn, Ym receiving the focus light from satellite 18 andproducing a boresight tracking error, in a conventional feedback controlsystem tracking loop 98, as shown in FIG. 7. Here, however, instead ofhaving the tracking loop 98 drive the fine steering mirror 31 to nullthe boresight error signal and thereby drive the boresight, or opticaxis of the optical system to point at the satellite 18, i.e., point inthe direction of the light 90 transmitted by satellite 18, the requiredlead angle, (as computed by computer 100, in a conventional manner usingconventional geometric equations) is added to the tracking signalproduced by the tracking loop 98 with the result that the optical axisof the optical system is directed to the expected position of satellite18 at the subsequent time t2, as shown in FIG. 6. Because theacquisition, tracking and communication lasers are all aligned with theoptic axis, the laser beams produced by such laser will be directed tothe satellite 18 at its expected position at time t2. Thus, the trackingloop 98 tracks with a finite boresight error signal, i.e., the leadangle, L. With this arrangement, a single mirror, here fine steeringmirror 31, is used during all phases (i.e., the acquisition, trackingand communication phases). That is, by using a spatially resolveddetector (i.e., here a CCD device, which provides a signalrepresentative of the actual position of the received light energyrelative to the boresight, or optic axis) the tracking loop 98 is ableto maintain the focused energy at a fixed lead angle, L, off of theboresight, or optic axis, as shown in FIG. 8, thereby eliminating theneed for a separate “point ahead” mechanism for the laser transmitters.

[0044] To put it another way, the monolithic optical structure 34 (FIG.6) provides an interface between the beam director, here fine beamsteering mirror 31 thereof, and the laser transmitter 22, 24, 26 andlaser energy detector 28. The monolithic optical structure has an opticaxis, or boresight axis, passing between the beam director, here mirror34 thereof, and the laser transmitter 22, 24, 26 and passing between thebeam director/mirror 34 and the laser energy detector 28. Incomingenergy from a source of laser energy, satellite 18, moving relative tothe tracking, or control system 96 is directed by beam director/mirror31 and the optical system 34 to the laser energy detector 28 along theoptic axis and the laser energy being produced by the laser transmitter22, 24, 26 is directed through the optical structure 34 and the beamdirector/mirror 31 along the optic axis to the source 18. The monolithicoptical structure 34 directs the incoming laser energy to a position onthe laser energy detector 28 (i.e., the focal plane of the CCD array ofpixel, FIG. 8) related to the angular deviation between the direction ofthe optic axis and the direction of the incoming laser energy. Computer100 (FIG. 7) computes the lead angle, L, between the present directionto the source 18 (i.e., the direction 101 (FIG. 6) between satellite 16and satellite 18 at time t1) and an expected direction to the source 18(i.e., the direction 103 between satellite 16 and satellite 18 at timet2. The control system 96 (FIG. 7) is responsive to a signal producedthe laser energy detector 28 related to the position of the incominglaser energy on such detector relative to the optic axis (FIG. 8) andthe computed lead angle, L, for tracking the source of incoming laserenergy with a tracking error related to the computed lead angle anddirecting the optic axis along the expected direction 103 (FIG. 6) tothe source 18.

[0045] Here, the acquisition field of view is 1 milliradian. Here a200×200 pixel CCD detector permits tracking accuracy on the order of 1micro radian. If the ratio of acquisition field of view to trackingaccuracy is on the order of 1000 to 1, then the same detector may beusable for both the acquisition and tracking modes. In such case, thetracking loop 98 is adapted to have a selected one of two bandwidths; aslower responding (I.e., smaller) bandwidth during the acquisition modeand a larger bandwidth during the tracking mode. Such dual modeoperation is represented diagrammatically in the tracking loop by a pairof amplifier-shaping networks G1, G2, respectively; amplifier-shapingnetwork G1 being switch into the loop 98 during the acquisition mode andamplifier-network G2 being switched into the loop 98 during the trackingmode.

[0046] Other embodiments are within the spirit and scope of the appendedclaim. For example, while the system has been shown for use withsatellite-satellite cross links and ground station up-links, the lasercommunication system could be used in satellites which provide groundsurveillance information developed by instrumentation carried on-boardthe satellite.

What is claimed is:
 1. A laser communication system adapted for use in asatellite, such laser communication system comprising: a beam director;laser transmitter/receivers; a monolithic structure comprising aplurality of active and passive optical elements for interfacing betweenthe beam director and the laser transmitters/receivers, laser energybeing directed between the beam director and the transmitters/receiversby the active and active optical elements, such laser energy passingthrough the monolithic structure solely as collimated light.
 2. Thelaser communication system recited in claim 1 wherein the monolithicstructure is configured to provide all optic axes between the beamdirector and laser transmitter/receivers in substantially a commonplane.
 3. The laser communication system recited in claim 2 wherein thelaser transmitter/receivers include an acquisition laser transmitter andan acquisition receiver used to enable the satellite to link up withanother satellite, or ground station, during an acquisition mode, and acommunication laser transmitter and an communication receiver adapted toenable the satellite to exchange data with the linked up satellite, orground station during a communication mode.
 4. The laser communicationsystem recited in claim 3 wherein the monolithic structure is configuredto dispose the optic axis between the beam director and laseracquisition and communication lasers and the optic axes between the beamdirector and the acquisition and communication receivers insubstantially a common plane.
 5. The laser communication system recitedin claim 4 wherein the laser transmitter/receivers include: a trackinglaser transmitter for enabling the linked up satellite, or groundstation to track the satellite during a tracking mode; and a singledetector use by the satellite during both the acquisition mode and asubsequent tracking mode.
 6. The laser communication system recited inclaim 5 wherein the single detector includes a charge coupled device. 7.The laser communication system recited in claim 1 including acollimating/beam shaping module, such module comprising a pair ofsubmodular units, a first one of the submodular unit including one ofthe transmitting lasers and a beam shaping lens, the second one of thesubmodular units having a collimating lens, the first and secondsubmodular units being aligned with, and affixed to, each other toprovide the collimating/beam shaping module, the collimating/beamshaping module being affixed to the monolithic structure.
 8. The lasercommunication system recited in claim 7 including a collimating/beamshaping module filter disposed on an surface of the second submodularunit.
 9. The laser communication system recited in calm 8 wherein thefilter protrudes beyond the second submodular unit and is provided witha surface adapted to interface, and be affixed to, a surface portion ofthe monolithic structure.
 10. A method of assembling a lasercommunication system adapted for use in a satellite, such lasercommunication system comprising: a beam director; lasertransmitter/receivers; and a monolithic structure comprising a pluralityof active and passive optical elements for interfacing between the beamdirector and the laser transmitters/receivers, laser energy beingdirected between the beam director and the transmitters/receivers by theactive and active optical elements, such laser energy passing throughthe monolithic structure solely as collimated light, such methodcomprising the steps of: forming a first submodular unit, such unitcomprising one of the lasers and a properly aligned beam shaping lens;forming a second submodular unit, such second unit having a mountedcollimating lens; aligning the first and second submodular units witheach other to form a collimating/beam shaping module; and affixing thecollimating/beam shaping module to a surface of the monolithicstructure.
 11. The method recited in claim 10 including the step ofproviding a filter on an surface of the second submodular unit.
 12. Themethod recited in claim 11 wherein the filter protrudes beyond thesecond submodular unit and is provided with a surface adapted tointerface, and be affixed to, the surface of the monolithic structure.13. A laser communication system adapted for use in a satellite, suchlaser communication system comprising: a transmitting laser, responsiveto electrical signals for converting such electrical signals intocorresponding light signals; a detector adapted to receive light signalstransmitted by a laser and convert such light signals into acorresponding electrical signals; a beam director adapted to directlight signals transmitted by the satellite to a receiver external to thesatellite and to direct light signals received by the satellite from asource external to the satellite; and, a monolithic optical structure,for passing therethrough the light signals from the transmitting laserto the beam director and for passing therethrough light signals receivedby the beam director to the detector, such monolithic optical structurecomprising a plurality of glass cubes and planar thin film bondedtogether and disposed to pass such light signals therethrough as onlysubstantially collimated light.
 14. The laser communication systemrecited in claim 13 including, additionally, a redundant lasertransmitter and wherein the thin films are responsive to electricalcontrol signals to direct the light signals from either the firstmentioned laser transmitter or the redundant laser transmitter throughthe monolithic optical structure to the beam director selectively inaccordance with the control signals.
 15. The laser communication systemrecited in claim 14 including an second pair of laser transmitters, andwherein the thin films are responsive to electrical control signals todirect the light signals from either a first one of the second pair oflaser transmitters or a second one of the second pair of lasertransmitters thorough the monolithic optical structure to the beamdirector selectively in accordance with the control signals.
 16. Thelaser communication system recited in claim 15 including a third pair oflaser transmitters, and wherein the thin films are responsive toelectrical control signals to direct the light signals from either afirst one of the third pair of laser transmitters or a second one of thethird pair of laser transmitters thorough the monolithic opticalstructure to the beam director selectively in accordance with thecontrol signals.
 17. The laser communication system recited in claim 16wherein one of the three pairs of laser transmitters is used during anacquisition mode, a second one of the three pairs of laser transmittersis used during a tracking mode, and a third one of the three pairs oflaser transmitters is used for communication of data during the trackingmode.
 18. The laser communication system recited in claim 17 includingan additional detector, and wherein the thin films are responsive toelectrical control signals to attenuate light signals from the beamdirector to either one of the detectors selectively in accordance withthe control signals developed by the detectors and coupled to the thinfilms in a feedback loop.
 19. The laser communication system recited inclaim 18 wherein optical paths between the three pairs of transmittinglasers and the beam director and between the beam director and the pairof detectors are disposed in substantially a common plane.
 20. The lasercommunication system recited in claim 19 wherein light signalstransmitted by the transmitting laser in one of the three pairs thereofand the light signals transmitted by the transmitting laser in a secondone of the three pairs are at different wavelengths.
 21. The lasercommunication system recited in claim 20 wherein laser signals passedfrom the beam director to a first one of the pair of detectors andlasers signals passed from the beam director to a second one of the pairof beam director have different wavelengths.
 22. The laser communicationsystem recited in claim 21 wherein the optical paths of two of the threepairs of transmitting lasers pass through a portion of the monolithicstructure in the same direction, wherein the optical paths of the thirdpair of transmitting lasers pass through one portion of the monolithicstructure in a direction perpendicular to the aforementioned direction.23. The laser communication system recited in claim 1 including a beamshaping/collimating laser transmitter module attached to the monolithicoptical structure, such laser transmitter module comprising: a firstsubmodular unit having a laser transmitter of the lasertransmitter/receivers and a beam shaping lens affixed to such lasertransmitter; a second submodular unit having affixed thereto acollimating lens for collimating a beam produced by the lasertransmitter; wherein the first and second submodular units are alignedwith, and affixed to, each other to provide the collimating/beam shapingmodule.
 24. The system recited in claim 23 including a collimating/beamshaping module filter disposed on an surface of the second submodularunit, such filter being provided with a surface adapted to interface,and be affixed to, a surface portion of the monolithic opticalstructure.
 25. The system recited in claim 24 wherein the lasertransmitter module is adapted to provide a beam of collimated light tothe diffraction limit.
 26. The system recited in claim 24 the firstsubmodular unit includes a submount for the laser transmitter.
 27. Thesystem recited in claim 26 wherein a microlens is affixed to thesubmount.
 28. The system recited in claim 27 wherein the beam shapinglens is a microlens bonded in close proximity to a laser emitting facet.29. The system recited in claim 28 wherein the microlens is a asphericrod microlens for anamorphic correction of the laser beam.
 30. Thesystem recited in claim 29 including a thermistor bonded to the uppersurface of the submount.
 31. The system recited in claim 32 includes alaser energy detector disposed to receive a small fraction of the laserenergy produced by the laser transmitter passing through the opening tosuch detector.
 32. The system recited in claim 31 including athermoelectric cooler disposed between the laser transmitter and a heattransfer device.
 33. The system recited in claim 32 including asubstrate and electronic driver circuitry supported on the substrate andelectrically coupled to the laser transmitter.
 34. The system recited inclaim 33 wherein the first modular unit comprises a hermetically sealedpackage.
 35. The system recited in claim 34 wherein the substrate hasdisposed on an upper surface thereof the laser energy detector.
 36. Thesystem recited in claim 35 wherein the first submodular unit includes asapphire window mounted to the package.
 37. The system recited in claim35 wherein the first modular unit is filled with an inert gas.
 38. Thesystem recited in claim 34 wherein the thermistor measures thetemperature of the laser transmitter and provides a feedback controlsignal to the cooler to provide temperature control for the lasertransmitter.
 39. The system recited in claim 38 wherein material usedfor the package has a thermal expansion coefficient matched to thethermal expansion coefficient of the substrate.
 40. The system recitedin claim 39 wherein the material used for the package has a thermalexpansion coefficient matched to the thermal expansion coefficient ofthe sapphire window.
 41. A laser transmitter module, comprising: a firstsubmodular unit having a laser transmitter and a beam shaping lensaffixed to such laser transmitter; a second submodular unit havingaffixed thereto a collimating lens for collimating a beam produced bythe laser transmitter; wherein the first and second submodular units arealigned with, and affixed to, each other to provide the collimating/beamshaping module.
 42. The module recited in claim 41 including acollimating/beam shaping module filter disposed on an surface of thesecond submodular unit.
 43. The module recited in claim 42 wherein thelaser transmitter module is adapted to provide a beam of collimatedlight to the diffraction limit.
 44. The module recited in claim 41wherein the first submodular unit includes a submount for securing thelaser transmitter.
 45. The module recited in claim 44 wherein one end ofthe laser transmitter is mounted to the laser submount.
 46. The modulerecited in claim 45 wherein the beam shaping lens is a microlens bondedin close proximity to a laser emitting facet.
 47. The module recited inclaim 46 wherein the microlens is a aspheric rod microlens foranamorphic correction of the laser beam.
 48. The module recited in claim47 including a thermistor bonded to the upper surface of the lasersubmount.
 49. The module recited in claim 48 wherein the firstsubmodular unit includes a laser energy detectors disposed to receive asmall fraction of the laser energy produced by the laser transmitterpassing through the opining to such detector.
 50. The module recited inclaim 41 including a thermoelectric cooler disposed between the lasertransmitter and a heat transfer device.
 51. The module recited in claim50 including a substrate and electronic driver circuitry supported onthe substrate and electrically coupled to the laser transmitter.
 52. Themodule recited in claim 51 wherein the first modular unit comprises atwo piece hermetically sealed package.
 53. The module recited in claim52 wherein the substrate has disposed on an upper surface thereof thelaser energy detector.
 54. The module recited in claim 55 wherein thefirst submodular unit includes a sapphire window mounted to the package.55. The module recited in claim 53 wherein the first modular unit isfilled with an inert gas.
 56. The module recited in claim 53 wherein thethermistor measures the temperature of the laser transmitter andprovides a feedback control signal to the cooler to provide temperaturecontrol for the laser transmitter.
 57. The module recited in claim 56wherein material used for the package has a thermal expansioncoefficient matched to the thermal expansion coefficient of thesubstrate.
 58. The module recited in claim 57 wherein the material usedfor the package has a thermal expansion coefficient matched to thethermal expansion coefficient of the sapphire window.
 59. The systemrecited in claim 1 wherein the transmitter/detectors include a singledetector for an acquisition mode and a tracking mode and wherein thecommunication system includes a control system, fed by the detector, toproduce control signals for positioning the beam director.
 60. Thesystem recited in claim 59 wherein the control system includes atracking loop fed by the single detector and a lead angle computer fordriving the optic axis of the monolithic optical structure to apredetermined tracking error, such tracking error being related to alead angle computed by the lead angle computer.
 61. A tracking systemfor tracking a source of incoming laser energy moving relative to thetracking system and directing a beam of laser energy to such source,such system comprising: a beam director; a laser transmitter a laserenergy detector; a optical structure having an optic axis passingbetween the beam director and the laser transmitter and passing betweenthe beam director and the laser energy detector, the incoming energybeing directed by beam director and the optical system to the laserenergy detector along the optic axis and such laser energy beingproduced by the laser transmitter being directed through the opticalstructure and the beam director along the optic axis to the source, theoptical system directing the incoming laser energy to a position on thelaser energy detector related to the angular deviation between thedirection of the optic axis and the direction of the incoming laserenergy; a computer for computing a lead angle between the presentdirection to the source and an expected direction to the source; and acontrol loop, responsive to: a signal produced the laser energy detectorrelated to the position of the incoming laser energy on such detectorrelative to the optic axis; and, the computed lead angle, for trackingthe source of incoming laser energy with a tracking error related to thecomputed lead angle and directing the optic axis along the expecteddirection such source.
 62. A laser communication system adapted for usein a satellite, such laser communication system comprising: a beamdirector; laser transmitter a laser energy detector; a monolithicstructure comprising a plurality of active and passive optical elementsfor interfacing between the beam director and the laser transmitter andlaser energy detector, such monolithic optical structure having an opticaxis passing between the beam director and the laser transmitter andpassing between the beam director and the laser energy detector,incoming energy from a source of laser energy moving relative to thetracking system being directed by beam director and the optical systemto the laser energy detector along the optic axis and such laser energybeing produced by the laser transmitter being directed through theoptical structure and the beam director along the optic axis to thesource, the optical system directing the incoming laser energy to aposition on the laser energy detector related to the angular deviationbetween the direction of the optic axis and the direction of theincoming laser energy; a computer for computing a lead angle between thepresent direction to the source and an expected direction to the source;and a control loop, responsive to: a signal produced the laser energydetector related to the position of the incoming laser energy on suchdetector relative to the optic axis; and, the computed lead angle, fortracking the source of incoming laser energy with a tracking errorrelated to the computed lead angle and directing the optic axis alongthe expected direction such source.